1. Field of the Invention
The present invention relates to an optical material for optical elements such as refractive optical elements and diffractive optical elements. More specifically, the present invention relates to an optical material with high dispersion (wide variation of refractive index with wavelength), and optical elements, diffractive optical elements and laminated diffractive optical elements molded therefrom.
2. Related Background Art
In conventional refractive optical systems based on solely light refraction, chromatic aberration has been reduced by combination of glass materials different in dispersion characteristics. For example, in a telescope, the chromatic aberration of the object lens is corrected by combining a positive lens made of a low-dispersion glass material and a negative lens made of a high-dispersion glass material. Thus, it is difficult to obtain sufficient correction of chromatic aberration when there is limitation in the configuration and number of available lenses or in available glass materials.
SPIE Vol. 1354 (International Lens Design Conference (1990)) discloses a method for reducing color aberration by using a diffractive optical element provided with a diffraction grating on the lens surface or as part of the optical system. This method utilizes a physical phenomenon that the chromatic aberration to light of a reference wavelength occurs in the opposite direction between a refracting surface and a diffracting surface of optical elements. Furthermore, by changing the period of the periodic structure of the diffraction grating, such a diffractive optical element can achieve the same effect as an aspherical lens. Accordingly, such diffractive optical elements are very effective to reduce chromatic aberration.
Here the diffraction effect of light is explained. In general, when a light beam enters into a spherical lens or an aspherical lens, i.e., an refraction based optical element, it comes out as a light beam even after refraction. On the other hand, a diffractive optical element splits the incident beam into plurality beams of respective orders by diffraction.
Accordingly, for full exertion of the optical feature of a diffractive optical element in an optical system, it is necessary to concentrate the light beam in a working wavelength region in a particular order (hereinafter it may be called “design order”). When the light beam of the working wavelength is concentrated in a particular design order, the intensity of the diffracted light in the other diffraction orders becomes so weak that light beams in the other orders will not give images (flare light) other than the image of the light beam in the design order.
JP09-127321A, JP09-127322A, JP11-044808A and JP11-044810A disclose structures where grating structure is determined so as to concentrate the light beam in the working wavelength region in a design order in order to obtain sufficiently high diffraction efficiency. These structures are constructed by combining a plurality of optical elements, in such a manner that diffraction efficiency becomes high over a wide wavelength range, choosing optimally dispersion of each optical element and the grating shape formed in the boundary region between the optical elements. Specifically, a desired diffractive optical element is formed so that a plurality of optical elements are laminated on a substrate, and at least one of interfaces is provided with a relief pattern, a staircase pattern, a kinoform or the like.
In these prior arts, to obtain a structure of high diffraction efficiency over a wide wavelength range, two materials are used in combination, those having relatively low and high dispersion respectively. Specifically, in JP09-127321A, BMS81 (nd=1.64, νd=60.1: from Ohara Inc.) is used as a material of low dispersion and a plastic optical material PC (nd=1.58, νd=30.5: from Teijin Chemicals Ltd.) is used as a material of high dispersion. Similarly, in JP09-127322A, LaL14 (nd=1.698, νd=55.5: from Ohara Inc.), acrylic resin (nd=1.49, νd=57.7), Cytop (nd=1.34149, νd=93.8: from Asahi Glass Co., Ltd.) are used as a material low in dispersion, and a plastic optical material PC (nd=1.58, νd=30.5: from Teijin Chemicals Ltd.) as a material high in dispersion. In JP11-044808A and JP11-044810A, C001 (nd=1.525, νd=50.8: from Dai-Nippon Ink and Chemical Industry Co., Ltd.) and PMMA (nd=1.4917, νd=57.4), and BMS81 (nd=1.64, νd=60.1: from Ohara Inc.) are used as a material of low dispersion, and a plastic optical material PC (nd=1.58, νd=30.5: from Teijin Chemicals Ltd.) and PS (nd=1.5918, νd=31.1) and the like are used as a material of high dispersion.
The larger is the difference in dispersion between these materials, the higher is the diffraction efficiency and the wider is the field angle of the optical element made thereof. For that purpose, it is necessary to use a material higher in dispersion (smaller in Abbe number); the use of such a material permits precise correction of chromatic aberration.
FIG. 1 is a graph showing the Abbe numbers and refractive indexes of commercially available optical materials. In FIG. 1, the ordinate axis represents the refractive index (nd) and the abscissa axis represents Abbe number (νd). FIG. 1 includes the optical materials described in the above-mentioned JP09-127321A, JP09-127322A, JP11-044808A and JP11-044810A. As can be see from FIG. 1, the refractive indexes of general optical materials satisfy the condition that nd>−6.667×10−3νd+1.70. Incidentally, the straight line shown in FIG. 1 represents the expression that nd=−6.667×10−3νd+1.70. Among the organic polymer optical materials shown in FIG. 1, polyvinylcarbazole (PVCZ) has the smallest Abbe number of 17.3.
However, the mere use of a material high in dispersion (small in Abbe number) is not sufficient for further function improvement of a diffractive optical element because there occurs some partial reduction of the diffraction efficiency in the working wavelength region even if the overall diffraction efficiency in the whole visible region is increased. FIG. 2 is a graph showing the diffraction efficiency of a multilayer diffractive optical element for which polyvinylcarbazole is utilized as the material high in dispersion. In FIG. 2, the ordinate and abscissa axes represent the diffraction efficiency and wavelength, respectively. As FIG. 2 shows, in the working wavelength region (from 400 nm to 700 nm), the diffraction efficiency is as low as in the order of 95% in the regions of 400 to 420 nm and of 630 to 700 nm; particularly, the diffraction efficiency is low in the shorter wavelength region. Accordingly, further improvement is needed.